2001ApJ...558L.101R

The Wisconsin Hα Mapper Northern Sky Survey has revealed a loop of H II reaching 1300 pc from the Galactic midplane above the Cassiopeia OB6 association in the Perseus spiral arm. This enormous feature surrounds and extends far above the ``W4 chimney'' identified by Normandeau, Taylor, and Dewdney and appears to be associated with the star formation activity near the W3/W4/W5 H II region complex. The existence of this ionized structure suggests that past episodes of massive star formation have cleared the H I from an enormous volume above the Perseus arm, allowing Lyman continuum photons from O stars near the Galactic midplane to reach into the halo.